Reversibility polyurethane

This simple reaction is the bedrock of the polyurethane iadustry (see Urethane polymers). Detailed descriptions of the chemistry and process have been published (65—67). Certain carbamates are known to reversibly yield the isocyanate and polyol upon heating. This fact has been commercially used to synthesize a number of blocked isocyanates for elastomer and coating appHcations. 

The pseudocross-links, generated by the hard-segment interactions, are reversed by heating or dissolution. Without the domain crystallinity, thermoplastic polyurethanes would lack elastic character and be more gum-like in nature. In view of the outlined morphology, it is not surprising that many products develop their ultimate properties only on curing at elevated temperature, which allows the soft- and hard-phase segments to separate. 

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

From these and other experiments a construction was developed that included a core of nonionic hydrophilic polyurethane. The core served as a reservoir for ionized salt. A layer of anionic gel was placed on one side of the core a layer of cationic gel was placed on the opposite site. A current passed through the gel caused the cation to migrate into the anionic gel and led the gel system to contract. When the current flow was reversed, the ions returned to the reservoir. 

Polyurethanes based on the HDI cyclic trimer show a rubberlike mechanical behavior. In fact, the Z1030/1072 and Z1031-H films show a low elastic modulus E, no yielding, and an ultimate, widely reversible, deformation beyond 100-150%, likely underestimated owing to the difficulty in assessing the ultimate properties of self-supported thin films. 

Some investigators who studied polyurethane synthesis proposed that both the forward and the reverse reactions were important. For example this approach was assumed for the formation of linear polyurethane,49 and the following kinetic equation was derived ... 

Pure MDI melts at 38°C. The MDI tends to dimerize readily, so it has to be stored at -4°C and melted just prior to use. The pure MDI can be modified so that it is liquefied at room temperature. The physical properties of the polyurethanes made from these modified isocyanates are inferior to the material made from pure MDI. The reversibility of the reaction is shown in Figure 2.17 (Dow Plastics, 2001). 

As the temperature decreases, the degradation effect on the polyurethane chain also reduces to virtually nil. Physical properties change, but these changes are reversible. The major changes are ... 

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